The therapeutic efficacy of cytotoxic anti-cancer drugs can be improved by increasing their selectivity towards cancer cells. One way to achieve this goal is to link a drug covalently to a monoclonal antibody or other type of targeting agent that has selective affinity towards the tumor cells. This approach has been extensively explored with various types of cytotoxic agents, and several types of humanized (or human) antibody-drug conjugates, or immunoconjugates, have been developed that combine high potency and selectivity in killing target tumor cells, long circulation retention times, and lack of immunogenicity. This new generation of highly effective anti-cancer agents includes conjugates of antibodies with maytansinoids, analogues of CC1065, derivatives of taxol (taxoids), of auristatin, and of a calicheamycin γ1. Several maytansinoid (DM1) conjugates are currently in clinical development, and a conjugate of an anti-CD33 antibody with calicheamycin, Mylotarg, has been approved by the Federal Food and Drug Administration for clinical use as an anti-cancer drug.
Many reports have been published detailing attempts to specifically target tumor cells with monoclonal antibody-drug conjugates (Sela et al. in Immunoconjugates 189-216 (C. Vogel, ed. 1987); Ghose et al, in Targeted Drugs 1-22 (E. Goldberg, ed. 1983); Diener et al, in Antibody Mediated Delivery Systems 1-23 (J. Rodwell, ed. 1988); Pietersz et al, in Antibody Mediated Delivery Systems 25-53 (J. Rodwell, ed. 1988); Bumol et al, in Antibody Mediated Delivery Systems 55-79 (J. Rodwell, ed. 1988). Cytotoxic drugs such as methotrexate, daunorubicin, doxorubicin, vincristine, vinblastine, melphalan, mitomycin C, and chlorambucil have been conjugated to a variety of monoclonal antibodies. In some cases, the drug molecules were linked to the antibody molecules through an intermediary carrier molecule such as serum albumin (Garnett et al. Cancer Res. 46:2407-2412 (1986); Ohkawa et al. Cancer Immumol. Immunother. 23:81-86 (1986); Endo et al. Cancer Res. 47:1076-1080 (1980)), dextran (Hurwitz et al. Appl. Biochem. 2:25-35 (1980); Manabi et al. Biochem. Pharmacol. 34:289-291 (1985); Dillman et al. Cancer Res. 46:4886-4891 (1986); Shoval et al. Proc. Natl. Acad. Sci. 85: 8276-8280 (1988)), or polyglutamic acid (Tsukada et al. J. Natl. Canc. Inst. 73:721-729 (1984); Kato et al. J. Med. Chem. 27:1602-1607 (1984); Tsukada et al. Br. J. Cancer 52:111-116 (1985)).
The efficacy of an antibody-drug conjugate is generally compromised by the heterogeneous expression of the target antigen in some tumors, because targeted delivery of conjugated drug via antibody binding to the antigen-positive tumor cells will spare any antigen-negative tumor cells to which the antibody cannot bind. Furthermore, tumor vasculature is an attractive target that is attracting more and more attention as a way to shrink tumors by starving them of their blood supply. Tumor vasculature is usually tumor antigen-negative. It has been reported previously that several classes of antibody-targeted cytotoxic agents are indeed capable of eradicating not only antigen-positive cells but also antigen-negative neighboring cells, a so called “bystander effect”. This bystander effect was observed with immunoradionuclides, immunoliposomes, and ADEPT (Allen T M, Nat Rev Cancer 2 (10):750-63 (2002); Muldoon L L, Neurosurgery 53(6):1406-12 (2003), Muldoon L L and Neuwelt E A, J Neurooncol 65(1):49-62 (2003)). In an abstract published in Proceedings of the American Association for Cancer Research, vol 41, March 2000, the authors describe that a portion of the cleaved DM1 is effluxed by p-glycoprotein and other efflux transporters in a cell. This is because, in the extracellular environment, the free released DM1-thiol readily reacts with cellular molecules having a sulfhydryl group or disulfide bonds, such as in cysteine or cystine, oxidized or reduced glutathione, or proteins (e.g., plasma albumin). Once reacted with these hydrophilic biological molecules, the DM-1 becomes an inactive, mixed disulfide compound, which precludes it from being effective to penetrate neighboring cells and to retain its potency.
Therefore, it would greatly benefit the treatment of various proliferative diseases if cell binding agent-drug conjugates could be designed in such a way so as to provide a more stable and potent form of the free drug to destroy not only the antigen-positive tumor cells, but also any surrounding antigen-negative tumor cells or cells of the neoplastic vasculature, thereby maximizing success in treating tumors by release of stable free drug from the conjugate inside the target cell.